Most conventional semiconductor power transistors are almost exclusively formed using silicon (Si). Due to the relative maturity of the use of this semiconductor, the performance of conventional power transistors to carry high currents and block high voltages is closely approaching the theoretical limit for Si. For example, power MOSFETs made using Si have undergone many improvements over the past two decades allowing them to block 30 to 600 volts, while providing relatively low on-state resistance values.
However, there are many applications for power devices that require the ability to carry high currents and block voltages in the range of 300 V to 5 kV (and greater). These applications include motor control, power supply, lighting ballast, power transmission and generation, and utility power conversion. Unfortunately, the overall performance of power devices made using Si is poor for this voltage range, and the lack of such power devices represents the primary limitation in realizing circuits for these applications. In fact, if high voltage devices that support such high currents and operate at frequencies of one to 1 MHz were available, they would revolutionize power utility applications and result in power savings of as much as $2 billion in the United States.
One recent development in semiconductor power devices is the use of Intelligent Power Modules (IPMs). IPMs use low voltage CMOS circuitry that may be integrated with power devices. Other examples of intelligent power devices include discrete integrated power devices that detect unacceptable current, voltage, and temperature conditions. However, the relatively low blocking voltage of semiconductor power devices made using Si limits the application of these devices in majority carrier devices (e.g., devices that rely on resistive current transport) to 300 V or less.
For higher power devices (e.g., those blocking voltages greater than 300 V), bipolar devices, such as, insulated gate bipolar transistors (IGBTs) and Thyristors have been used. While these devices offer acceptable on-state performance, they suffer from relatively slow switching speeds and poor performance at high temperatures.
Other power devices that have been researched also suffer from various deficiencies. For example, Bipolar Junction Transistors (BJTs) use a current control gate rather than a preferable voltage control gate. Many vertical junction field effect transistors (JFETs) operate in a “normally-on” mode during their on-state condition; however, JFETs with the preferable “normally-off” mode have poor on-state resistances. Finally, thyristors have high on-state voltage drops (because of their inherent junction drop) and slow switching speeds.
A variety of power devices using silicon carbide (SiC) have been researched and implemented in an attempt to provide devices that block high voltages and carry high currents. One switching power device is the vertical power MOSFET. However, vertical power MOSFETs made using SiC suffer from poor performance and poor reliability because of low inversion layer channel mobility. In addition, the tunneling current between SiC and the gate dielectric of power devices made using SiC limits their reliability during long term operation. Unlike Silicon and Silicon Carbide, vertical Gallium Nitride (GaN) based power MOSFETs are not considered feasible presently due to the unavailability of native GaN substrates.
One example of a high power microwave device formed using GaN is the AlGaN/GaN HEMT. A HEMT is operated by controlling the source to drain current through modulating a two dimensional electron gas via a widebandgap AlGaN gate located between these two terminals. However, these devices suffer from poor reliability due to the high defect levels at the hetero-junction growth of AlGaN layer above the unoped GaN layer. Other devices that may be formed using GaN devices are GaN MESFETs and JFETs.